This invention relates to head loading systems, and more particularly, to a head loading system of a magnetic disc apparatus or flexible disc apparatus, capable of moving toward and away from an information recording medium at least one magnetic head for at least reproducing information recorded on such information recording medium.
Generally a magnetic disc apparatus or flexible disc apparatus comprises a head loading system for moving at least one magnetic head serving as an information reproducing head toward and away from a magnetic disc functioning as a recording medium for recording information therein. The head loading system comprises a magnetic head support mechanism for supporting the magnetic head in juxtaposed relation to one surface of the magnetic disc, and a head loading actuator for driving the magnetic head support mechanism to move the magnetic head toward and away from the surface of the magnetic disc. A magnetic head support mechanism of a magnetic head loading system of the prior art will be described by referring to FIGS. 1(a) to 2(b).
The magnetic head support mechanism of the head loading system shown in FIGS. 1(a) and 1(b) comprises a gimbaled spring 4 for resiliently supporting a magnetic head 1a jaxtaposed against a top surface of a magnetic disc 6, a swing arm 3a for supporting the gimbaled spring 4 for vertical movement and supporting a hook 5 at one end portion and a downwardly projecting cam 7a at the other end portion, a swing arm 3b mounting at one end portion a magnetic head 1b in juxtaposed relation to a bottom surface of the magnetic disc 6 and having at the other end portion an upwardly extending cam 7b which is brought into engagement with the cam 7a, a support portion 50 for supporting the swing arms 3a and 3b through support springs 2, respectively, and preloading springs 8a and 8b extending from the support portion 50 to urge the swing arms 3a and 3b respectively to move in a direction in which they are closed. The numeral 9 designates a bail for moving the hook 5 vertically which constitute a part of the magnetic head loading actuator subsequently to be described.
When the magnetic heads 1a and 1b are respectively located close to the top and bottom surfaces of the magnetic disc 6, the magnetic head support mechanism is located such that, as shown in FIGS. 1(a) and 1(b), the bail 9 has its forward end 9a spaced apart from the hook 5 because it is located in a lower position. As the swing arms 3a and 3b are pressed by the preloading springs 8a and 8b respectively, the magnetic heads 1a and 1b are moved to positions in which they are close to the top and bottom surfaces of the magnetic disc 6 respectively. In the head loading system of the aforesaid construction, the head loading actuator is operative to cause the forward end 9a of the bail 9 to lift the hook 5 to thereby cause the magnetic head 1a supported by the swing arm 3a through the gimbaled spring 4 to move away from the top surface of the magnetic disc 6, as shown in FIGS. 2(a) and 2(b). As the cam 7a moves leftwardly to press the cam 7b of the arm 3b to move it leftwardly in the figure in the process of the movement of the swing arm 3a in the clockwise direction, the swing arm 3b moves in the counterclockwise direction to move the magnetic head 1b supported by the arm 3b away from the magnetic disc 6.
Upward movement of the bail 9 causes the parts in positions shown in FIG. 1(a) to shift to positions shown in FIG. 2(a). The operation characteristic of the bail 9 is very important because it would exert influences on (1) the speed of response of the head loading system to an external signal and (2) the damage which the magnetic disc and the magnetic heads might suffer when the latter strike the former. Particularly in recent years, the head loading actuator of a head loading system is required to have the following functions:
(1) To respond quickly to an external signal and move the magnetic heads near to or into intimate contact with the magnetic disc in a very short period of time;
(2) To avoid damage which the magnetic disc might suffer when the magnetic heads strike same by causing the latter to come into contact with the former gently; and
(3) To perform the aforesaid operations with minimized energy. To cope with this situation, head loading systems of the prior art use a head loading actuator shown in FIG. 3. The head loading actuator shown in the figure comprises a push-pull solenoid 12 including a plunger 13, a coil 14 and a fixed pole 16, the bail 9 having one end adapted to come into engagement with the hook 5, and a return spring 11. The bail 9 has a central portion pivotally connected to the plunger 13 of the solenoid 12 and is adapted to come into engagement at the forward end 9a with the hook 5 while it is pullsed at its rear end by the return spring 11 to be pivoted at a pin 10. As an energizing current is passed to the coil 14, the solenoid 12 is energized and pulls the plunger 13 downwardly against the biasing force of the return spring. This causes the bail 9, pivotally connected to the plunger 13, to move in the clockwise direction about the pin 10 to move downwardly the hook 5 in engagement with the forward end 9a of the bail 9. Upon interruption of the supply of the energizing current to the coil 14, the bail 9 is moved in the counterclockwise direction by the biasing force of the return spring 11 about the pin 10 to move the hook 5 upwardly in the figure. By these operations, the head loading actuator moves vertically up and down the hook 5 of the magnetic head support so as to thereby move the magnetic heads toward and away from the magnetic disc.
FIG. 4 shows the relation between the stroke and the attracting force of the solenoid 12 that can be established when the energizing current passed to the coil 14 is constant. In FIG. 4 the figure, it will be seen that in the process of movement of the forward end of the plunger 13 from an upper-most position (starting point 15a) of a stroke of 3 mm to a lowermost position (terminating point 15b) of a stroke of 0, the attracting force increases rapidly non-linearly from about 0.3 kg to 2.5 kg as indicated by an arrow C. The phenomenon that the attracting force increases with a reduction in stroke would be accounted for by the fact that, as the plunger 13 is attracted by the magnetic force and moves downwardly as shown in FIG. 3, the gap .delta. decreases and the magnetic flux density in the gap .delta. increases while the area spacing the plunger 13 away from the coil 12 increases.
FIG. 5 shows the stroke/load characteristic of the head loading system in which the stroke represents an overall resilience of the system including the biasing force of the return spring 11 of the head loading actuator and the biasing forces of the preloading springs 8a and 8b of the head support mechanism. As the stroke decreases from a condition (starting point 15c) in which the forward end 9a of the bail 9 is in engagement with the hook 5, the load increases little by little until a point D is reached at which the magnetic heads are brought into contact with the surfaces of the magnetic disc and the bail 9 is released from engagement with the hook 5 when the load decreases once, but thereafter the load increases little by little as balance is restored between the load and the force of restitution of the return spring 11 until a terminating point 15d is reached.
FIG. 6 shows the characteristic of FIG. 4 combined with the characteristic of FIG. 5. As shown in FIG. 6, it is necessary that an attracting force curve 100 of the solenoid 12 be higher at all times than a load curve 200 to allow the magnetic heads to move away from the magnetic disc, and the solenoid 12 would be inoperative if the attracting force is low as indicated by an attracting force curve 101.
Attention is directed to FIGS. 4-6 in which the attracting force shown is obtained by continuously passing an energizing current of a constant value to the coil 12. In actual practice, the attracting force would show variations in a transition state in passing an energizing current of a constant value to the coil of an actual apparatus. The variations occurring in the attracting force in the transient state are as follows:
(a) Assuming the magnetic field in the gap .delta. between the plunger 13 of the push-pull solenoid 12 and the fixed pole 16 is H(AT/m), and the magnetic flux density and the area thereof is B(WB/m.sup.3) and A(m.sup.2) respectively. Then, the attracting force F (kg) can be expressed by the following equation: ##EQU1##
Assuming the magnetic permeability is .mu.o, then the magnetic flux density B can be expressed by the following equation: EQU B=.mu.o.H (2)
Thus, equation (1) can be rewritten as equation (3) as follows: ##EQU2##
Assuming the constant is determined by the construction of the solenoid 12, and number of turns of the coil 14 and the energizing current are K, N and I, respectively, then the magnetic field H can be expressed by the following equation: EQU H=K.N.I (4)
From equations (3) and (4), the attracting force F can be expressed by the following equation: ##EQU3##
Thus, when the constant determined by the construction of the solenoid 12 and the number of turns of the coil 14 is denoted by A.sub.o, it will be seen that the attracting force of the plunger of the solenoid of the predetermined shape is proportional to the square of the energizing current I as shown by the following equation: EQU F=A.sub.o.I.sup.2 ( 6)
where ##EQU4##
(b) However, when the inductance and the internal resistance of the coil 14 are denoted by L and R, respectively, and a step voltage E.sub.o is impressed thereon, the energizing current will rise with an inclination of L/R with time and draw near E.sub.o /R, as shown in the following equation and FIGS. 7(a) and 7(b): ##EQU5##
(c) Thus, as can be clearly seen in equations (6) and (7), a change with time of the attracting force in an actual apparatus has a characteristic such that the attracting force suddenly increases as indicated by an attracting force curve 110 shown in FIG. 8 as well as the following equation (8): ##EQU6##
The stroke/attracting force characteristic shown in FIG. 8 indicates that since the attracting force 110 rises suddenly the plunger 13 has a very high acceleration when the stroke is 0. Because of this, there are great possibilities that the magnetic heads 1a and 1b would be forced, by the very high acceleration, to strike the magnetic disc 6 to cause damage to both the magnetic disc 6 and the magnetic heads 1a and 1b, since in an apparatus of the prior art, the hook 5 in engagement with the forward end 9a of the bail 9 would have its movement greatly accelerated. Moreover, the head loading system of the prior art has the problems in that a stopper 17 strikes a top surface of the solenoid 12 with a high force and produces a large noise, and that the solenoid 12 has a high consumption of electric power because it is necessary to place a string point 15e shown in FIG. 8 in a relatively high position. Additionally, the solenoid is located perpendicular to the planes of surfaces of the magnetic disc in the head loading system of the prior art, making it necessary for the apparatus to have, in addition to the vertical dimension of the solenoid, a vertical dimension that would enable the bail 9 and hook to move vertically upwardly away from the upper end of the solenoid. Thus, it has been impossible to reduce the vertical dimension of the head loading system, and consequently difficulties have been experienced in obtaining a magnetic disc apparatus of small thickness.
A head loading actuator similar to the one shown in FIG. 3 is disclosed in Japanese Patent Application Laid-Open No. 58311/76 corresponding to U.S. Ser. No. 510,471, now U.S. Pat. No. 3,973,274 for example.
In order to obviate the problems raised by the head loading actuator of the head loading system of the prior art of the aforesaid construction, attempts have been made by us to adopt the following measures:
(i) The head loading actuator would have a solenoid which would, as shown in FIG. 9, be constructed such that the plunger 13 would have a lower end projecting downwardly in the form of a cone and the fixed pole 16 would have a shape complementary with the aforesaid shape of the lower end of the plunger 13, to thereby reduce as much as possible a sudden change in the attracting force by minimizing a sudden change in the magnetic flux density and the opposed surfaces of the plunger 13 and the coil 14.
(ii) The head loading actuator would have a construction such that the point of connection between the bail 9 and the plunger 13 would be moved from the position shown in FIG. 3 to a position closer to the pin 10 serving as the pivot for the bail 9, to minimize the influences which might be exerted by a sudden change in the attracting force by decreasing the stroke of the plunger 13.
(iii) The head loading actuator would have a construction such that the push-pull solenoid 12 would have an electrical damping function or a hydraulic damping function.
However, it has been ascertained that some disadvantages are associated with various constructions of the head loading actuator described hereinabove. The solenoid construction described in paragraph (i) has proved, upon experiments, to have no marked improvement in operation characteristic. The head loading actuator described in paragraph (ii) could achieve no excellent effects because the compactness of the apparatus makes it impossible to increase the lever ratio l.sub.0 /l.sub.1 (FIG. 3) of the bail 9. In the head loading actuator provided with a damper as described in paragraph (iii), the reliability of the apparatus as a whole would be lowered due to obturation of the damper and the short service life thereof, thereby making the apparatus of no practical value. Thus, it has been ascertained that no satisfactory operation characteristic can be obtained even if some improvements were provided in a head loading system of the prior art wherein the plunger and the bail are directly connected together.